Cable assemblies typically comprise one or more insulated conductors presented in a round bundle or in a flat ribbon. To avoid interference from electromagnetic radiation, commonly referred to as electromagnetic interference, EMI, and radio frequency interference, RFI, cable assemblies are overwrapped with a conductive shielding material, e.g. metal conduit, wire braid, metal foil, carbon-filled polymer or metallized fabric. A variety of shielding jackets for cables are disclosed in U.S. Pat. Nos. 3,089,915; 3,582,532; 4,281,211; 4,375,009; 4,376,229; 4,409,427; 4,461,076; 4,684,762 and Japanese Laid-Open Utility Model Application 4-66725. Shielding jackets for cable connectors and junctions are disclosed in U.S. Pat. Nos. 3,946,143; 4,016,356; and 4,865,892.
Wire braid is a common shielding material which is effective especially against low frequency interference. At high frequency, where the wavelength of the radiation begins to approach the size of the apertures in the shielding material, the leakage of radiation through apertures adversely affects shielding. Smaller apertures in braid are achieved by reducing wire size. However, minimum wire size is limited by the cost and difficulty of drawing fine wire. Consequently, braided wire shielding material, e.g. braid of 36 gauge tinned copper wire, begins to leak significantly at 1 to 10 megahertz (MHz).
In gigahertz range radar or in high speed data systems having fast rise time harmonics, there is a need to provide shielding effectiveness at both very low and very high frequencies with low weight materials. Often a combination of wire braid and metal foil is used to achieve the desired shielding with an undesirable weight of shielding materials. At higher frequencies, metal foils are especially effective in shielding where the shielding mechanisms are capacitive, inductive and reflective effects. Metal foils are difficult to produce in a low weight, flexible and durable thickness that is effective at both low and high frequencies. Thin metal foils are typically provided as laminates on a flexible plastic film. Overlapping seams of such laminates inherently allow leakage, e.g. with spiral wrapped shielding. Longitudinally wrapped metal foil laminates can be edge-folded to provide metal-metal contact reducing the leaking phenomena.
In many applications, e.g. in computer assemblies and aircraft, design of shielded cable assemblies often requires a compromise between desired level of shielding effectiveness and weight. An object of this invention is to provide lower weight shielded cable assemblies with an enhanced level of shielding effectiveness.